The present invention relates to a cutting tool geometry in general, and more particularly to the geometry of throw-away cutting inserts so designed as to favor or promote the formation of a false cutting tip build-up at the cutting edge of the cutting tool during normal operation.
Cutting tools made of hard, wear-resistant material, such as cemented metal carbides, generally in the form of throw-away cutting inserts mounted on the tip of an appropriate tool holder, are of common use today when heavy cuts and high cutting speeds are desired. Such cutting inserts are used in combination with appropriate means for controlling the snaking chip removed from the workpiece, so as to break the continuous chip into small individual pieces which fall to the floor or into an appropriate container, without interfering with the cutting edge of the cutting tool, or with the workpiece, and without endangering the machine operator.
The chip control element, or chip breaker, may consist of a separate member which is engaged with the top surface of the cutting tool, at a position remote from the cutting edge or cutting tip and which, once clamped in position, interferes with the free flow of the continuous chip and breaks it into small pieces. Alternatively, chip breakers may take the form of appropriate recesses and rises formed integrally in a cutting insert. Whatever the type of chip control used, there results an additional costly element incorporated into the tool holder or a complex geometry for the cutting insert which in turn causes an increase in manufacturing costs due to complex shaped molds for forming the "green" inserts, complications and breakage due to uneven shrinkage during sintering and difficulties in finish grinding the inserts after sintering.
During cutting of a chip from a workpiece, the cutting edge or tip of the tool is subjected to considerable stress and loads, and the material at the cutting edge itself and behind the cutting edge is subject to considerable thermal shock, intense heat and constant abrasion. A stream of coolant and lubricating fluid is generally directed as close to the cutting edge as is permissible under the particular machining conditions. However, because the cutting edge itself is generally masked by the chip being removed from the workpiece, and because of the snaking of the chip and of the random breakage of the chip into small pieces, a free flow path for the lubricant-coolant is unavailable, and the flow is constantly interfered with and interrupted, which further increases the repeated thermal shocks to which the cutting tool is subjected. All those adverse conditions result in rapid wear of the cutting tool, and rapid dulling of the cutting edge.
It has been observed that under all conditions of chip formation, a transfer of material particles is effected from the chip and the workpiece to the cutting edge and to the surface of the cutting tool proximate the cutting edge, such transfer of particles causing a false cutting tip to build upon the surface of the cutting tool from the cutting edge to a short distance behind the cutting edge. The formation of such a false cutting edge which builds up on the cutting tool is particularly pronounced when intense heat is generated at the cutting tip, and the particles of material forming the cutting edge build-up become literally welded and strongly adhering to each other, with the particles at the base of the build-up adhering to the surface of the cutting tool. Such build-up of a false cutting tip, which is continuously renewed during cutting of the chip, provides a protective element interposed between the surface of the curling chip and the surface of the cutting tool which prevents direct contact between the chip and the cutting tool surface and cutting edge, such that the life of the cutting tool is greatly improved as the false cutting tip build-up acts as a shield against abrasion of the cutting tool surface and cutting edge by direct contact with the chip, and also as a shield against thermal shocks.
It has been discovered by applicant that the formation of a false cutting tip build-up is greatly improved if the surface of the cutting tool behind the cutting edge is in the form of a very shallow groove, is rough rather than smooth, is coated with a thin layer of diamond particles adhering to the surface, or if the cutting tool is operating at a negative cutting angle. Either one of those conditions, or a combination of some or all of those conditions, greatly promotes the formation of a false cutting tip build-up on a cutting tool, as long as the prerequisite condition of avoiding as much as possible disturbing the formation of a free flowing chip is also met.